(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming contacts without metal thinning and pitting in the fabrication of integrated circuits.
(2) Description of the Prior Art
In metal dry/wet etching processes, aluminum thinning and pitting are major issues. When the metal pitch is scaled down and the topography becomes more variable, metal notching becomes a serious problem and can affect the reliability of the integrated circuit.
Referring to FIG. 1, there is shown a partially completed integrated circuit. Field OXide regions 11 have been formed in and on a semiconductor substrate 10. Polysilicon or polycide gate electrodes 14 have been formed. A dielectric layer 16, such as borophosphosilicate glass (BPSG) covers the gate electrodes. A barrier metal layer 18 is coated over the dielectric layer. Metal layer 20, typically aluminum, is deposited over the barrier metal layer 18. A layer of photoresist is coated over the metal layer 20 and patterned to form a mask. Because of the highly reflective surface of the metal layer 20, light rays 22 are reflected onto the photoresist layer causing overexposure of portions of the layer. The resulting photoresist mask has a notch in it. Therefore, when the metal is patterned according to the photoresist mask and etched, a metal notch 24 will result, as shown in top view in FIG. 2.
FIG. 3 illustrates the notching problem occurring in the second metal layer. FIG. 3 illustrates the same partially completed integrated circuit as in FIG. 1 wherein an antireflective coating 30 has been deposited over the first metal layer 20. Intermetal dielectric layer 32 covers the patterned first metal layer. Barrier metal layer 34 is deposited followed by second metal layer 36. A layer of photoresist is coated over the metal layer 36 and patterned to form a mask. Because of the highly reflective surface of the metal layer 36, light rays 38 are reflected onto the photoresist layer causing overexposure of portions of the layer. The resulting photoresist mask has a notch in it. Therefore, when the metal is patterned according to the photoresist mask and etched, a metal notch 40 will result, as shown in top view in FIG. 4. It is undesirable to employ an antireflective coating on the topmost metal layer because an undesired polymer is easily formed on the sidewalls of the via hole and some of the thickness of the topmost metal layer may be lost during stripping of the antireflective coating. It would be necessary to modify the etching process to completely remove an antireflective coating.
A further problem with the prior art in metal etching is illustrated in FIG. 5. The partially completed integrated circuit of FIG. 1 is shown after metal etching and after removal of the photoresist mask. Pits 25 appear on the surface of the patterned metal layer 20. These pits are the result of the chemical reaction between water and the metal during wet etching, especially if the metal is an aluminum copper alloy. The pitting problem is worst in the bonding pad area.
U.S. Pat. No. 5,126,289 to Ziger describes a method of coating the metal to be patterned with an organic antireflective coating and removing both the antireflective coating and the metal with a plasma etch. U.S. Pat. No. 5,017,513 to Takeuchi describes a method for using an organic layer mask pattern over a metal film.